This invention relates to interferometers for use in optical communication networks, and more specifically to optical signal interleavers/deinterleavers designed to produce greatly reduced amounts of chromatic dispersion.
In multiplexed optical communication networks, a single optical fiber typically carries multiple independent data channels with each data channel assigned to a different optical wavelength. Such networks are referred to as wavelength division multiplexed (WDM) networks. As signals propagate through the network, data for different channels may be separated or combined using an optical frequency filter, in particular, an interleaver/deinterleaver (hereafter xe2x80x9cinterleaverxe2x80x9d).
An interleaver is a type of optical multiplexer which, when operating as an interleaver, combines subsets of channels from different fibers into a single optical beam. When operating as a deinterleaver, the interleaver separates a single optical beam having a series of channels into two or more subset series of channels. Typically, an interleaver is used to separate or combine even and odd International Telecommunications Union (ITU) channels.
FIG. 1 conceptually illustrates the function of an interleaver. When operating as an interleaver, the interleaver receives a first optical beam 100, which comprises a number of even channels at frequencies f2, f4, f6. The frequencies of each channel are such that each of these channels is separated by the same amount, e.g. 100 GHz. The interleaver also receives a second optical beam 102, which comprises a number of odd channels at frequencies f1, f3, f5. Similar to beam 100, the frequencies of each of these channels are such that these channels are separated by the same amount, e.g. 100 GHz. The even and odd channels, however, are offset from each other, normally an amount equal to half their separation distances, e.g. 50 GHz. The interleaver then interleaves the beams 100 and 102 to generate a beam 104 with the channels f1, f2, f3, f4, f5, f6, which are separated by 50 GHz. When operated as a deinterleaver, beam 104 is received and divided into beams 100 and 102.
Optical frequency interleavers are widely recognized as key components enabling the rapid expansion of WDM networks to higher channel counts and narrower channel spacing, while preserving inter-channel cross-talk performance, in combination with existing demultiplexer technologies. Because of the periodic frequency nature of the International Telecommunications Union (ITU) grid, interleavers tend to be constructed from combinations of one or more interferometric structures, such as etalons and Mach-Zehnder interferometers. The desirable features of interleaver pass bands include a flattop and high isolation in the stop-band.
A Michelson interferometer uses a beamsplitter and two reflecting mirrors to separate wavelengths of a light signal into different optical paths. This type of interferometer provides a linear phase ramp dependent on the optical path difference between the two arms of the interferometer. The linear phase ramp generates a non-flat top response with no chromatic dispersion.
Another type of interferometer, invented by Dingel, is a Michelson interferometer in which the mirror of one arm is replaced by a Gires-Tournois (GT) etalon. As shown in FIG. 2, an interferometer 200 comprises a beam splitter 202 (typically an approximately 50/50 splitter), a plate 204 with a highly reflective (near 100%) coating 206 placed in one arm with spacers 207a and 207b preferably made from ultra low expansion material (ULE). A GT etalon 220 is placed in the other arm. The GT etalon 220 comprises a front plate 208 with a partially reflective (e.g., 15% reflectivity) coating 210, spacers 211a and 211b preferably made from ultra low expansion material (ULE) and a back plate 214 with a highly reflective (near 100%) coating 212. As shown, a gap of distance d separates front plate 208 and back plate 214 of the GT etalon 220. Further, the GT etalon 220 is placed a distance L2 from the beam splitter 202, and the plate 206 is placed a distance L1 from the beam splitter 202.
When this set-up is used in an interleaver for deinterleaving channels, an incident beam B1 comprising, for example, ITU even and odd channels is directed towards beam splitter 202. Beam B1 is split at splitter interface 222 into a beam B3 and beam B2. Beam B3 is directed towards plate 204 with highly reflective coating 206, while beam B2 is directed towards GT etalon 220. Because of the near 100% reflectivity of reflective coating 206, beam B3 is reflected back to splitter 202. Beam B3 experiences a linear phase change per wavelength based upon the distance traveled from the splitter interface to plate 204 and back. An exemplary linear phase ramp of beam B3 at splitter interface 222 is illustrated in FIG. 2c as line 242.
Likewise, because of the near 100% reflectivity of reflective coating 212, beam B2 is reflected back to splitter 202. However, in addition to experiencing a linear phase change per wavelength based upon the distance traveled, beam B2 also experiences a non-linear phase change from GT etalon 220 of,   Φ  =            -      2        ⁢                  tan                  -          1                    ⁡              [                                            1              -                              R                                                    1              +                              R                                              ⁢                      tan            ⁡                          (                                                2                  ⁢                  πη                  ⁢                                      xe2x80x83                                    ⁢                  d                                λ                            )                                      ]            
where R is the power of reflectance of coating 210, xcex is the vacuum wavelength and xcex7 is the refractive index of the material inside GT etalon 220. Typically, the material inside GT etalon 220 is air, resulting in a refractive index xcex7 equal to approximately 1. An exemplary non-linear phase ramp of beam B2 at splitter interface 222 is illustrated in FIG. 2c as line 240 for a 15% reflectivity of coating 210.
Therefore, when beams B2 and B3 meet at splitter interface 222, there is a resulting phase difference of,   ΔΦ  =                    4        ⁢        πΔ        ⁢                  xe2x80x83                ⁢        L            λ        +          2      ⁢                        tan                      -            1                          ⁡                  [                                                    1                -                                  R                                                            1                +                                  R                                                      ⁢                          tan              ⁡                              (                                                      2                    ⁢                    πη                    ⁢                                          xe2x80x83                                        ⁢                    d                                    λ                                )                                              ]                    
where the optical path difference xcex94L is the difference between the distance L1 and L2 (i.e., L1xe2x88x92L2).
The phase graphs illustrated in FIG. 2c result when MGTI 200 is designed such that the optical path difference, xcex94L, is one half, or multiples of one half, the GT air gap, d. As described, GT etalon 220 perturbs the linear phase ramp of the interferometer 200 and produces a non-linear phase ramp. When the optical path difference, xcex94L, is one half, or multiples of one half, the GT air gap, d, this non-linear phase ramp generates a flat top response function that is desired in telecommunication systems. For the case that xcex94L is one half the GT air gap, the phase difference between beam B2 and B3 when they meet at splitter interface 222 is,   ΔΦ  =                    2        ⁢        π        ⁢                  xe2x80x83                ⁢        d            λ        +          2      ⁢                        tan                      -            1                          ⁡                  [                                                    1                -                                  R                                                            1                +                                  R                                                      ⁢                          tan              ⁡                              (                                                      2                    ⁢                    πη                    ⁢                                          xe2x80x83                                        ⁢                    d                                    λ                                )                                              ]                    
When beams B2 and B3 meet at the splitter interface, part of beam B2 is reflected, while part of beam B3 is passed through, thereby forming beam B4. Referring to FIG. 2c, at the frequencies where these two portions are substantially 180xc2x0 (i.e. xcfx80) out of phase, destructive interference occurs, while constructive interference occurs at the frequencies where these two portions are substantially in phase. The interference between these portions of beams B2 and B3 result in beam B4 having a standard intensity pattern of,       I          (      t      )        =            I      o        ⁢                  sin        2            ⁡              (                  ΔΦ          2                )            
This spectral response is illustrated in FIG. 2b as line 230. This spectral response results in beam B4 carrying a first sub-set of channels (e.g., the even channels).
Also, when beams B2 and B3 meet at splitter interface 222, part of beam B3 is reflected with a phase change of xcfx80, while part of beam B2 is passed therethrough, thereby forming beam B5. Because the portion of B3 that forms B5 is reflected with a phase change of xcfx80 (i.e. 180xc2x0), the phase ramps for the portions of B1 and B2 that form B5 are similar to that shown in FIG. 2c, except phase ramp 242 is shifted by xcfx80. This changes the frequencies where the portions of B3 and B2 that form B5 are in phase and where they are out of phase. The interference between these portions of beams B2 and B3 results in beam B5 having a standard intensity pattern of,       I          (      r      )        =            I      o        ⁢                  cos        2            ⁡              (                  ΔΦ          2                )            
This spectral response is illustrated in FIG. 2b as line 232. This results in beam B5 carrying a second sub-set of channels (e.g., the odd).
Generally, the shape of the spectral responses of MGTI 200 is determined by the reflectivity of reflective coating 210, while the period between transmission peaks, i.e. the interleaver free spectral range (FSRint), of the spectral response is determined by the gap distance d of GT etalon 220. The FSRint is equal to c/(2xcex7d cos(xcex8)), where c is the velocity of light (e.g. 299792458 m/s), xcex7 is the refractive index of the cavity (e.g. xcex7air=1.000273), and xcex8 is the angle of incidence (e.g. 0xc2x0). Therefore, to provide an interleaver operable on systems having, for example, 50 GHz channel spacing, the gap distance of GT etalon 220 is adjusted to provide an FSRint of 50 GHz, i.e. d=2997.1 xcexcm.
Another type of interleaver (herein after referred to a xe2x80x9cSEIxe2x80x9d) is disclosed in U.S. Pat. No. 6,125,220 issued Sep. 26, 2000 to Copner et al, and U.S. Pat. No. 6,281,977 issued Aug. 28, 2001 in the name of J D S Fitel, which are both incorporated herein by reference. The disclosed interleaver combines the reflected and transmitted fields from a single etalon to provide interleaver/de-interleaver functions.
Presently, systems exists at 50 GHz channel spacing but this channel spacing is likely to decrease with time, resulting in a need for interleavers operable for systems at 25 GHz channel spacing in the near future. This requirement puts extremely tight constraints on the interleaver spectral pass shape. Although prior art MGTI systems produce useful spectra, the interleaving and deinterleaving capabilities may not be sufficient for 25 GHz, or lower, systems. Furthermore, the prior art systems fail to provide for an interferometer that has relatively low dispersion and capable of interleaving and deinterleaving 25 GHz, or lower, systems. Therefore, whatever the precise merits, features and advantages of the above described prior art systems, none of them achieve or fulfills the purposes of the present invention.
The present invention provides for an interferometer comprising a beamsplitter and two optical resonators, e.g. GT etalons or ring resonators. The beamsplitter splits an input beam of light into a first sub-beam directed to follow a first path and a second sub-beam directed to follow a second path. The first resonator has a first effective cavity length and receives the first sub-beam. The second resonator has a second effective cavity length and receives the second sub-beam. The first path and the second path have an effective optical path difference approximately equal to one-half the first effective cavity length.
In one embodiment, the reflectivities of the front plates of the GT etalons are different, and are selected to provide a desired spectral response. In the preferred embodiment, the ratio of the reflectivities of the front plates should range from between 8:1 to 30:1. Some examples of reflectivities include: 45% and 4.5%, 35% and 2.5%, etc. Furthermore, the higher front reflectivity is selected from between 10%-90%, and more preferably between 25%-60%. Additionally, the lower reflectivity plate varies from 1% to 10%.
In another embodiment, the two resonators are slightly de-phased from one another such that the dispersion slope of the first resonator is oppositely aligned, and preferably equal, with the dispersion slope of the second etalon, so that the overall dispersion of the device is greatly reduced.
Another aspect of the present invention relates to an interferometer with a predetermined free spectral range (FSR) comprising:
a polarization dependent delay section to produce an effective optical path difference of approximately L between orthogonally polarized components of an input beam of light; and
a resonator, optically coupled to said polarization dependent delay section, having a cavity length of substantially 2 L;
whereby, when the orthogonally polarized components of the input beam of light are recombined, a series of wavelength channels with a predetermined polarization pattern are formed.
Another aspect of the present invention relates to an interferometer with a predetermined free spectral range (FSR) comprising:
a optical resonator having a first port for launching an input beam of light, a second port for outputting reflected light from the resonator, a third port for outputting transmitted light from the resonator, and an effective cavity length; and
first coupling means for combining the reflected and transmitted light from the optical resonator, wherein the reflected and transmitted light have a first effective optical path length difference between the optical resonator and the first coupling means of approximately half the effective cavity length.
Preferably, the aforementioned interferometer further comprises second coupling means for combining the reflected and transmitted light from the optical resonator, wherein the reflected light and the transmitted light have a second effective optical path length difference between the of approximately +/xe2x88x92n(xcexc/4) or +/xe2x88x92n(FSR/2), wherein xcexc is the center wavelength of the input beam of light, and n is an integer.